Electrolytic refining process is mainly suitable for the electrolytic refining and purification of metals such as copper, lead, nickel and the like, in which a crude metal serves as an anode, a pure metal serves as a cathode, and a solution containing ions of this metal serves as an electrolyte; the metal dissolves from the anode and precipitates at the cathode. Among the impurities in the crude metal, the inactive ones do not dissolve but become anode slime; since the metal having a higher electrode potential precipitates preferentially at the cathode, and the electrode potential of each metal is determined by the standard electrode potential and the concentration of the metal ions, thus the active ones cannot precipitate at the cathode due to a lower ion concentration thereof, though it dissolve at the anode.
The metal in an electrolytic process follows Faraday's law. Taking copper as an example, its electrolytic precipitation amount may be represented by the following equation:mCu=n×1.1852×i×A×t  (1)
in the equation: mCu is the mass (g) of the precipitated metal, i is the current density (A/m2), t is the time (s), A is the area (m2) of the cathode plate, and n is the number of the electrolytic tank (1).
As can be seen from equation (1), on the premise of the existing process equipments and technologies, the only means to improve the productivity is to increase the current density. However, in the production practice, in case of simply increasing the current density, the metal precipitation at the cathode is accelerated, which tends to cause a decrease in the concentration of the metal ion Cu2+ near the cathode, namely, to generate the concentration polarization, thereby resulting in a decreased electrode potential, making the main metal unable to precipitate preferentially on the cathode, leading to the precipitation of the metal impurities and affecting the quality of the products. An increase in current density on the anode induces the anode to dissolve too fast, makes the Cu2+ produced from anode dissolution unable to leave the interface between the anode and the solution rapidly to diffuse toward the cathode region, leading to concentration polarization as well. If the Cu2+ concentration in the anode region reaches saturation or supersaturation, copper oxides or insoluble salts will be produced and deposited on the anode surface, which will retard the anode reaction, increase the anodic potential, and result in contamination of the electrolyte due to dissolution of a large quantity of the impurity ions into the electrolyte, in severe cases even result in anode passivation, thereby increasing energy consumption.
In addition, as for the anode plate with high impurity comprising a large quantity of impurities such as Pb, As, Sb, Bi, Ni and the like, a relatively thick layer of anode slime will be deposited on the surface of the anode plate during the electrolytic process, and its failure to settle timely will affect the migration and diffusion of Cu2+, and in severe cases will result in anode passivation. Consequently, concentration polarization and anode passivation are main factors that cause limitation to the increase in current density in the electrolytic refining process.
Consequently, how to provide an electrolytic process which is capable of eliminating concentration polarization and avoiding occurrence of anode passivation phenomenon becomes a major technical problem that needs to be solved urgently by those skilled in the art.